1. Field of the Invention
The present invention relates to an image processing device which performs a multi-valued coding process on document images containing photographic images using an error dispersion method to reproduce the gradations of the images with high fidelity.
2. Description of the Related Art
In general, in image processing devices which handle not only character code information but also image information read from an original by a read means such as a scanner, high-contrast image information, such as characters, lines, etc., is subjected to simple binarization (binary coding) using a fixed threshold value, whereas image information such as photographs is subjected to binarization by a pseudo-gradation-reproducing means such as a dithering scheme. The reason is that, with the simple binarization process using a fixed threshold value, no image degradation occurs in character/line image areas because image resolution is preserved, whereas image degradation occurs in photographic image areas because the gradations thereof cannot be preserved. On the other hand, when a gradation reproducing means such as organizational dithering is performed on image information read from an original to reproduce gradations thereof, photographic image areas are not degraded because the gradations thereof are preserved, whereas character/line image areas are degraded because their resolution is lowered. That is to say, with a single binarization process for image information including photographs, characters and lines, it is impossible to ensure the quality of each of image portions which differ in feature at a time.
As a binary coding technique which ensures the gradations of photographic image portions and moreover provides good resolution for character/line image portions in comparison with the organizational dithering technique an error dispersion method has been proposed. For information on this method, reference is made to "An adoptive Algorithm for spatial gray scal" by R. W. Floyd and L. Steinberg, Proceeding of the SID, Vol. 17, pp 75-77, 1976. According to this error dispersion method, binary-coding errors of peripheral pixels already binary coded (or multi-valued or multilevel coded) are multiplied by weighting coefficients, the values of the products are added to the density value of a pixel of interest, and then the pixel of interest is binary coded with a fixed threshold value. Here, the binary-coding error refers to a difference between input and output signals of a binary coding circuit. In other words, the error dispersion method is the one to disperse a binary-coding error produced by binary coding a pixel of interest to peripheral pixels for error compensation, thereby minimizing binary-coding errors. In the case where an input image is a photograph, therefore, it is possible to perform a binary coding process which ensures the gradations of the photograph.
Even when the error dispersion method is used, however, it is required to make gamma corrections on printer's recording characteristics on the basis of the density by the use of a correcting means. Since, even if binary (two-valued) signals thus obtained, and high level "1" and low level "0" are output by a printer, the reflectance of each pixel recorded is not accurately 100% or 0%, thus, the conventional method cannot perform accurate error calculations. Since input signals to the system based on the error dispersion method are signals based on reflectance, error corrections, error calculations, and error filtering should be performed on the basis of reflectance signals.